It's not necessary to use lens hood all the time but there are some benefits of using it. Lets discuss what are those benefits and you can decide whether you want to use it or not.

Benefits of using the lens hood

First thing is it helps to remove stray lights from different sources. It prevents the light from hitting the front elements of the lens, which will reduce the contrast and create flare caused by the excess light, from the sides. Generally, pictures taken using the lens hood are clearer with rich colors, balanced contrast and the deeper saturation.

Lens Hood

Second purpose of using the lens hood is to protect your lens. We spend few hundreds to thousands of dollars buying lenses and may be it is a good idea to protect it against any accidental physical damage or scratch on the front elements of the lens. Lots of professional photographers and personally, I also use the lens hood most of the time to protect my lenses. Good quality lens hoods are made out of strong and good quality plastic which can prevent your lens from accidental scratch or damage. And the good thing is, it doesn't cost much if it doesn't already come with your lens. You can buy the lens hood from $10 to $100 depending upon the quality and the average price is around $30. So why not spend that much money to protect your several hundreds or thousand of dollars worth lenses, right?

When not to use the lens hood?

This is my personal experience and may not be an issue with other photographers but I found an extra shadow in the photographs when I use the lens hood in the dark area and shooting with the built-in flash. I think it's probably because of the lens I was using (18-200mm) and since the height of the built-in flash is not much, the hood might have blocked the part of the flash and caused a strange shadow on my photographs. If you are using a wide angle lenses or short prime lenses, you might be able to use it without any such problem.

Some photographers do not like to use the lens hood when they are using the polarizing filter. But I use an UV filter and the lens hood all the time together and I have had no issues shooting with it in a bright day light condition.